Controlling electrolytically generated gas bubbles in in-plane electroosmotic pumps

ABSTRACT

An “in-plane” electroosmotic pump may reduce deterioration of performance due to electrolytic gas generation. By controlling the flow of gas generated at the electrodes, while allowing ionic current, the gas may be prevented from fouling the narrow slots which act as pumping channels.

BACKGROUND

This invention relates generally to electroosmotic pumps and in particular to “in-plane” electroosmotic pumps. These are pumps where fluid flow is induced in multiple slots formed in a planar structure.

Existing in-plane electroosmotic pumps that produce relatively high flow rates are prone to formation of gas bubbles. These bubbles result from electrolytic decomposition of the pumping fluid at the pump electrodes. As an example, if the pumping liquid is water, hydrogen gas is produced at the cathode and oxygen gas is produced at the anode. These bubbles displace the fluid in the pumping channels of in-plane electroosmotic pumps, reducing pumping performance after a short period of time. Bubbles can also lead to poor electrochemical coupling.

Ultimately, the effectiveness of high flow rate in-plane electroosmotic pumps is severely limited by the presence of the bubbles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged, horizontal, cross-sectional view of one embodiment of the present invention;

FIG. 2 is an enlarged, partial cross-sectional view taken generally along the line 2-2 in accordance with one embodiment of the present invention;

FIG. 3 is a cross-sectional view taken generally along the line 3-3 in FIG. 1 in accordance with one embodiment of the present invention;

FIG. 4 is a schematic depiction of one of the electrodes shown in FIG. 1 in accordance with one embodiment of the present invention;

FIG. 5 is a schematic depiction of the other electrodes in accordance with one embodiment of the present invention; and

FIG. 6 is a schematic depiction of a system in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, an electroosmotic pump 10 may be fabricated in silicon or other semiconductor material, in one embodiment, using semiconductor fabrication techniques. The pump 10 is capable of pumping a fluid, such as a cooling liquid, through a row of slots 20 formed in a semiconductor. In one embodiment, the row of slots 20 may be formed by either wet or dry etching techniques. A pair of opposed electrodes 32 may generate an electrical field that results in the transport of a liquid through the row of slots 20. In one embodiment, the electrodes are fabricated from platinum. All areas of the pumping surface may be coated in an insulating material to prevent current leakage through the conducting semiconductor substrate. In one embodiment, the insulating material may be silicon dioxide, silicon nitride or multiple layers of these materials.

This process by which fluid pumping occurs is known as the electroosmotic effect. In such a case, hydrogen from the hydroxyl groups on the walls of the slots 20 deprotonate, resulting in an excess of protons near the wall surface. The excess hydrogen ions move in response to the electric field applied between the electrodes 32 in the direction of the arrows A (from anode to cathode). The non-charged water atoms also move in response to the applied electric field because of the drag forces that exist between the ions and the water atoms.

As a result, a pumping effect may be achieved without any moving parts in some embodiments. In addition, the structure may be fabricated in silicon at extremely small sizes, making such devices applicable as pumps for cooling integrated circuits and many other applications.

Referring to FIG. 2, which shows a cross-section of the row of slots 20 in FIG. 1, the row of slots 20 may be composed of a series of vertical walls 54 separated by trenches 56 which define a plurality of parallel channels for fluid and charge flow between the electrodes 32. The electrode 32 a may act as the anode 28 a and the electrode 32 b may act as the cathode 28 b.

Also provided in the liquid W may be a buffer which adjusts the pH of the liquid. In one embodiment, sodium borate may be used as a buffer to improve the zeta potential which is a measure of the excess ion charge near a solid surface in the fluid. For example, 0.5 mM of sodium borate buffer may be utilized in water.

Relatively high flow rates may be achieved in some embodiments. However, eventually, the flow rates diminish in conventional embodiments because of the displacement of the fluid by gas in the narrow channels by bubbles produced at each of the electrodes 32.

Thus, referring to FIG. 5, at the anode 28 b, oxygen gas is generated by the electrode 32 b. At the cathode 28 a, shown in FIG. 4, hydrogen gas is generated. These gases could eventually fill the surrounding area and, ultimately, displace the fluid in the trenches 56 in the row of slots 20.

Referring to FIG. 1, in order to contain the bubbles and to prevent them from being entrained within the row of slots 20, a closed, tubular sheath 30 may be provided around each electrode 32 to form the anode 28 b and cathode 28 a. The sheath 30 may be made of a material that passes liquid and ions or charge, but blocks bubbles and gas. Instead, the collected gas inside the sheath 30 passes outwardly of the pump 10 through an appropriate material 34. That is, when the gas pressure builds up inside the sheath 30, the gas passes outwardly through the material 34. Thus, not only is the gas prevented from fouling the row of slots 20, but excess gas is discarded from the system.

While many proton exchange membranes may be used for the sheath 30, in some embodiments, the sheath 30 may be a Nafion brand material made by E.I. DuPont de Nemours & Co. of Wilmington, Del. The specific form of Nafion® material used in some embodiments is a tube which may be obtained from Perma Pure LLC of Toms River, N.J. 08754.

Nafion® material is a copolymer of perfluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid and tetrafluoro-ethylene. Thus, Nafion® material has a Teflon® backbone with side chains of another fluorocarbon. Those side chains may terminate in a sulfonic acid. The Nafion® material may function as an ion exchange resin. Each sulfonic acid group may absorb up to thirteen molecules of water. The sulfonic acid groups create, effectively, ionic channels through the polymer so that water is very readily transported through the channels, while gas is not.

In some embodiments, a doubled tube of Nafion® material may be utilized as the sheath 30 to better contain the gas. In addition, spacers 40 may be provided between the electrodes 32 and the sheaths 30 to prevent gas outflow. It has been found by the present inventors that if the electrodes 32 contact the sheaths 30, gas may escape. Thus, spacers 40 may be provided along the length of each electrode 32 to space the sheath 30 away from the electrode 32. In one embodiment, the spacers 40 may be formed of globules of epoxy adhesive attached to the electrode surface.

The material 34, which allows the gas to flow outwardly of the pump 10, may be Gortex® brand fabric. The material 34 prevents loss of pumping liquid while allowing gas to escape outwardly from the electroosmotic pump 10. In one embodiment, the electrodes 32 may simply pass through the material 34. In another embodiment, a Nafion® tube may be connected to a manifold block that contains the material 34.

In some embodiments, relatively high flow rates (such as high as 10 milliliters per minute per square centimeter of planar pumping structure) with high pressures (such as 0.5 pounds per square inch) may be obtained. These flow rates may be continuous in that they are not prone to substantial bubble fouling in some embodiments.

The row of slots 20 may be patterned and etched to form an individual pump semiconductor die 12. The wafer 12 may consist of walls 59 (FIG. 1), liquid filled reservoirs 62 (FIG. 1), and a row of trenches 56 (FIG. 2) which act as the pumping medium. The pump wall 59 may be between 0.1 and 1 millimeter thick in one embodiment. The trenches 56 may have a depth between 10 and 300 microns, in some embodiments, with a width between 1 and 10 microns and a length between 5 and 100 microns.

The pump walls 54 (FIG. 2) between trenches 56 may have a width between 5 and 100 microns in one embodiment. A slot 60 (FIG. 1) may also be etched in the wall 12 of each liquid reservoir 62 at the same time the trenches 56 are formed. A sheathed electrode 32 is then inserted into each slot 60 in a post-silicon processing step. The slot 60 width may be on the order of 0.1 to 1 millimeter in one embodiment.

The pump die 12 may be coated with an insulating liner material 58 (FIG. 2) prior to electrode 32 insertion. The liner material 58 insulates the walls 54 to prevent current leakage into the conductive silicon and achieves an appropriate final slot 56 width (by reducing the slot width). Appropriate materials for covering silicon walls 54 include, without limitation, oxide produced by thermal oxidation, low pressure chemical vapor deposition silicon nitride, low pressure chemical vapor deposition polysilicon silicon that is then oxidized.

Referring to FIG. 3, the die 12 may be covered by a cover plate 26. The cover plate 26 may have holes 36 formed in it to communicate with tubing connectors 22. Liquid may be drawn into the pump 10 and expelled from the pump 10 through the connectors 22. The cover plate 26 may, for example, be formed of glass or silicon, to mention a few examples. For example, and without limitation, bonding techniques, such as anodic bonding, may be used for glass, metallic bonding for silicon, or direct bonding for silicon may be used to bond the cover plate 26 to the die 12. Typically, the tubing connectors 22 may be secured by adhesive 24, such as epoxy.

The electrodes 32 may be formed of platinum and are inserted within the sheaths 30. The sheathed electrodes 32 are then inserted into the electrode slots 60 (FIG. 1) that span the length of a trench row and run parallel to the length of the slots 20. Close proximity of the electrodes 32 to the row of slots 20 improves pump performance, as this reduces resistive losses. That is, the electrode-to-trench row potential drop is reduced. However, if the sheath 30 is positioned too close to the trench row of slots 20 or if the sheath 30 tube diameter is almost the same dimension as the reservoir 62 height, fluid flow may be restricted.

The material 34 may be a polytetrafluoroethylene or Gortex® brand membrane that allows the gas from inside the tube to escape while trapping water inside the sheath 30. The sheath 30 traps electrolytic gases inside the sheath. Electrolytic gas generated within the sheaths 30 may not enter the pump reservoirs 62, thereby interfering with the electroosmotic pumping action. Outside the pump 10, the electrode 32 passes through either the membrane 34 at the end of the sheath 30 or through the sheath 30 tube itself or into a manifold. An electrode via may be sealed in place with an adhesive, such as epoxy, in some embodiments.

Thus, referring to FIG. 6, the electroosmotic pump 10 may be supplied with a potential as indicated. In some embodiments, the liquid may be pumped to a mechanical cooler 48 that cools a semiconductor integrated circuit 50. The flow then proceeds through channels 44 to a radiator 46 which removes excess heat in some embodiments.

In some cases, a semiconductor package 52 may be formed with the pump 10, cooler 48, integrated circuit 50 to be cooled. The integrated circuit may be a microprocessor, for example. Then, the radiator 46 may be secured by conventional techniques to the package 52. However, the present invention need not be limited to semiconductor cooling embodiments.

References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 

1. A method comprising: controlling gases formed by an electrode of an electroosmotic pump while allowing the passage of liquid.
 2. The method of claim 1 including surrounding the electrode with a membrane which passes ions but controls gases.
 3. The method of claim 2 including surrounding said electrode with a proton exchange membrane.
 4. The method of claim 3 including providing spacers between a tubular membrane and said electrode.
 5. The method of claim 2 including providing a port to eject gases from within said membrane.
 6. The method of claim 5 including providing a membrane over said port, which membrane passes gas but prevents liquid from passing.
 7. The method of claim 6 including providing a membrane including Gortex® fabric.
 8. The method of claim 1 including passing a liquid through a series of slots in said pump.
 9. The method of claim 8 including forming a pair of reservoirs on either side of the series of slots in a semiconductor die.
 10. The method of claim 9 including forming a sidewall around said reservoirs and forming a slot at the top of said sidewall to receive an electrode.
 11. An electroosmotic pump comprising: a pair of electrodes; a plurality of liquid passages between said electrodes; and a sheath around at least one of said electrodes which sheath passes ions and reduces the passage of gas.
 12. The pump of claim 11 wherein said sheath is a proton exchange membrane.
 13. The pump of claim 11 including a semiconductor die with reservoirs formed therein, and passages formed between said reservoirs, said electrode positioned in one of said reservoir.
 14. The pump of claim 13 including slots in said die to receive said electrode.
 15. The pump of claim 12, said sheath including a doubled tube of proton exchange membrane material surrounding said electrode.
 16. The pump of claim 11 including spacers to space said sheath from said electrode.
 17. The pump of claim 11 wherein said sheath is tubular and has a material which allows gas from within the sheath to pass outwardly of said pump while preventing liquid from within said sheath from passing through said material.
 18. The pump of claim 17 wherein said material includes Gortex® brand fabric.
 19. The pump of claim 17 wherein said material is positioned over said sheath on the outside of said pump.
 20. The pump of claim 19 wherein said electrode passes through said material.
 21. The pump of claim 19 wherein the passage of said electrode through said material is sealed with adhesive.
 22. A system comprising: an integrated circuit; a cooler associated with said circuit; and an electroosmotic pump coupled to said cooler, said pump including an electrode covered by a sheath to pass ions and to control gas.
 23. The system of claim 22 including a heat exchanger coupled to said pump.
 24. The system of claim 22 wherein said cooler, said integrated circuit, and said pump are formed in one integrated circuit package.
 25. The system of claim 24 wherein said sheath includes a proton exchange membrane.
 26. The system of claim 25 wherein said sheath is a Nafion® tube.
 27. The system of claim 22 including a semiconductor die with reservoirs formed therein, and slots formed between said reservoirs, and electrodes in said reservoirs.
 28. The system of claim 22 including spacers to space said sheath from said electrode.
 29. The system of claim 22 wherein said sheath is tubular and has a material which allows gas from within the sheath to pass outwardly of said pump while preventing liquid from within said sheath from passing through said material.
 30. The system of claim 29 wherein said material includes Gortex® fabric. 